I am going to meet with a homeowner this after noon and he is concerned about Humidity levels in the house. This is what he has shared with me about his house so far: 

That sounds great.  Look forward to it.  I work close to our home so could meet you fairly quickly with some advanced notice.

I think there are two keys with our mostly brick veneer house...the attic and basement.  Today, we are currently sitting at right about 50% humidity on the first and second floors.  When it got in the upper 80s last week, humidity was 55% upstairs and 52% downstairs, which is just brutal especially after spending $10K on a new HVAC system from (JOHN DOE) HVAC.

Some of the questions I have in my mind and would like to talk to you more about in person...

-  With an air handler in the attic and flexducts, what extra precautions does a homeowner need to take to condition the second floor as efficiently as possible?  Should all the flex duct lines be wrapped in more insulation or buried in blown insulation keeping in mind we have a plywood walkway to our air handler.
-  With the vents in the second floor ceiling located close to the exterior of the home, is there a way to seal that air so drafts do not come in from attic through those vents into our bedrooms?  I have used spray foam insulation between the cut out of the ceiling vents and the drywall.
-  Should we consider a radiant barrier to keep as much weather out of the attic?
-  We added some vents in the attic but don't know if enough of the soffits are open and circulating the air properly
-  With a non-walk out basement, what ways are there to best seal a 70% finished basement? 

Thanks again. 

I am clearly going to be looking for the apparent air leaks in the house and the insulation levels... I am thinking that  there may be a problem with the sizing of the HVAC system.  What are your thoughts with regards to where the real problems are in this house?

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I would look primarily at the "70% finished basement" --did they seal the CMU before putting up drywall? What about the floor?  If they have ceramic tile in the basement, efflorescence is a good indicator of moisture movement through the floor(That could be present on bare concrete & CMU also). See what your blower door, IR, and moisture meter tell you. Look for signs of mold in and behind the drywall. Did the HO take before & after pics of the basement?

A close look at the 30% unfinished may give some clues as well. And, of course, a good inspection of the exterior for the dead giveaways like rain gutters, mildew, depressions near the foundation, etc. 

Unfortunately there is some missing information here, which of course you'll have a chance to gather. I also would take a look at  the basement and what the humidity levels are there. It's interesting that he gave you the RH without the temperatures.

The HVAC system could certainly be an issue, but beware that over-sizing is not the only possible issue. In order for an HVAC system to properly dehumidify basically two things have to happen: the coil temperature has to be below dewpoint and the system has to run long enough. (I have a post on a blog I've just started to address some of those issues - I'd be glad to pass it along if you like.)

Another issue with a basement is that even if it has AC installed typically it will have very short run times - if it is used at all. An HVAC system can't dehumidify if it isn't running, so they may have to install a whole house dehumidifier.

Thanks for the advice Bruce and Kevin,

I had a gut feeling that the HVAC system has been over sized so I asked my HVAC guy to the house to check for those issues. I also conducted a blower door test on the house and found that that there was .44 ACH.  So there is room for tightening up the house but I am waiting for the results of the Manual J Calculation before I meet with the home owner to discuss the next step and make recommendations for improving the home.

Running the blower at a lower speed can help with humidity issues. A simple humidistat hooked to the "dehum" terminal on the furnace board can help.

Something to remember is that newer high SEER AC's have a higher coil temp than the unit it replaced.  This reduces the ability to dehumidify. If you compare a new A coil you will see that they are larger than old A coils.  More surface area but warmer temps.  Like another poster said lower fan speeds may help.

An AC takes about 10 minutes to reach peak efficiency. I would see how long the run times are.

The other aspect is the source of the humidity. Are the owners using exhaust fans. Air leaks can contribute a large amount of the humidity. In the basement tape some foil or plastic wrap to the concrete floor and wall.  See which side collects moisture to see the source of the moisture.

On the outside check drainage. Down spouts and clogged gutters are obvious places. Check the slope for drainage.  What is often overlooked is what is hiding under the mulch or river rock next to the foundation. Many time it covers over low spots that hold water.  Particularly in newer homes mulch/rock is placed next to the foundation. As the ground settles next to the house low spots develop but are hidden.  I always pull back the mulch/ rock next to the house. I put a foot next to the house and then take my over foot and dig it down to dirt about 3 feet out.  You will be able to tell if there is positive slope away from the house.

The rim joist area in a finish basement is hard to access. Textured ceilings are hard to patch and add to the cost. If there is vinyl siding it may be possible to come from the outside and inject foam. But in the summer the big air leaks may be from the top. As we cool air is becomes denser and heavier. It sinks from top to bottom.  Unlike winter this makes the basement the high pressure zone.  

But maybe the biggest concern for me in this house are the ducts and air handler in the attic. Any air leaks from the supply side in the attic are going to depressurize the home sucking in warm moist air. The take off from the trunk lines will probably be the greatest source of air leaks. There is also the possibility there are return side leaks from the attic.I would say a duct blaster test is in order.

I think using a radiant barrier may be in order to shield the air handler.  I would pile the insulation up high on the ducts after they have been thoroughly sealed. 

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